This invention provides automatic selection of the correct total resistance for proper loading in a high voltage measurement circuit.
Robust monitoring of elevator signals require proper setting of voltage thresholds, as well as proper loading of unwanted noise. If the voltage threshold is set too low, invalid detection of coupled noise will result; if the voltage threshold is set too high, valid signal detection will be blocked. In addition, too little of loading will not squelch coupled noise, and too much loading will cause excessive power being drawn from the monitored signal (which could impact elevator operation). Previous digital input designs require an elevator installer to know ahead of time, or measure beforehand, the voltage of each signal, and then manually select the proper threshold and loading through wire jumpers as illustrated in FIG. 1. Therein, an opto-sensor 9 senses the voltage across a shunt resistor 10, the voltage being stepped down significantly by means of a series resistance 11 which is selected from a plurality of resistors 12-15 by means of wire jumpers 16-18. In other known measuring systems, the proper threshold and loading are established by selecting the proper input module to plug into a xe2x80x9cback-planexe2x80x9d board. This manual approach leaves room for human error and takes a significant amount of time to measure and set.
Objects of the invention include sensing high voltage signals with proper thresholds and loading without the use of wire jumpers, back-plane boards, or other manual selection; reducing the risk of human error in the measurement of high voltage signals; eliminating human involvement in establishing proper threshold and loading for measuring high voltage signals; reducing the time required to install an elevator having a monitoring system; and providing a voltage monitoring system that automatically adjusts itself as conditions change.
According to the present invention, each sensor of a high voltage signal system is provided with an over-current detector to provide a signal indicative of the fact that the system is drawing too much current; proper threshold and loading is provided by automatically increasing the resistance in series with the signal sensing resistor to the point of eliminating the over-current condition. In accordance further with the invention, whenever the monitoring circuit is turned off, all of the sensors are set in their maximum resistance condition so as to minimize loading of the elevator circuitry. In still further accord with the invention, when the monitoring circuitry is first turned on, it is preconfigured with the help of elevator installation personnel, by causing each elevator circuit which is monitored to first assume an on state and then assume an off state, while at the same time the automatic monitoring system is monitoring overcurrent and establishing proper settings for each resistance, in turn. Thereafter, each time the monitoring system is turned on, having once been preconfigured, it will initialize each resistance at the resistance determined to be correct during preconfiguration, or determined by automatic adjustment, during operation. Then, as the elevator operates, the automatic monitoring system will respond to an overcurrent at any of the sensors to increase the resistance at that sensor, store the new setting, and optionally generate an alarm message.
The invention reduces risk of human error through mis-measurement or misplaced jumpers, and saves installation time by not requiring installation personnel to measure signals and place jumpers on a circuit board or modules in a back-plane board. The invention adjusts the measuring circuit automatically, over time.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.